Cannot copy object constructed from default-constructed array

Question

After 1 or 2 hours spent to isolate a compilation error surrounded by a metaprogramming mess generating awfull compilation messages, here is a minimal and simple example that illustrates my problem:

#include <iostream>
#include <type_traits>
#include <array>
#include <utility>
#include <tuple>

template <class Crtp, class... Types>
struct Base
{
    Base(const Types&... rhs) : 
        data(std::forward_as_tuple(rhs...)) {;}
    std::tuple<Types...> data;
};

struct Derived 
: public Base<Derived, std::array<double, 3>>
{
    template <class... Args> 
    Derived(Args&&... args) :
        Base<Derived, std::array<double, 3>>(std::forward<Args>(args)...) {;}
};

int main(int argc, char* argv[])
{
    Derived a(std::array<double, 3>({{1, 2, 3}})); 
    Derived b(a);
    Derived c(std::array<double, 3>()); 
    Derived d(c); // Not working : why ?
    return 0;
}

This is compiled with g++ 4.8.1 and I do not understand exactly why the compiler complain when I try to copy c in d and not a in b.

Here is the error:

main.cpp: In instantiation of ‘Derived::Derived(Args&& ...) [with Args = {Derived (&)(std::array<double, 3ul> (*)())}]’:
main.cpp:28:16:   required from here
main.cpp:20:73: error: no matching function for call to ‘Base<Derived, std::array<double, 3ul> >::Base(Derived (&)(std::array<double, 3ul> (*)()))’
         Base<Derived, std::array<double, 3>>(std::forward<Args>(args)...) {;}
                                                                         ^
main.cpp:20:73: note: candidates are:
main.cpp:10:5: note: Base<Crtp, Types>::Base(const Types& ...) [with Crtp = Derived; Types = {std::array<double, 3ul>}]
     Base(const Types&... rhs) : 
     ^
main.cpp:10:5: note:   no known conversion for argument 1 from ‘Derived(std::array<double, 3ul> (*)())’ to ‘const std::array<double, 3ul>&’
main.cpp:8:8: note: constexpr Base<Derived, std::array<double, 3ul> >::Base(const Base<Derived, std::array<double, 3ul> >&)
 struct Base
        ^
main.cpp:8:8: note:   no known conversion for argument 1 from ‘Derived(std::array<double, 3ul> (*)())’ to ‘const Base<Derived, std::array<double, 3ul> >&’
main.cpp:8:8: note: constexpr Base<Derived, std::array<double, 3ul> >::Base(Base<Derived, std::array<double, 3ul> >&&)
main.cpp:8:8: note:   no known conversion for argument 1 from ‘Derived(std::array<double, 3ul> (*)())’ to ‘Base<Derived, std::array<double, 3ul> >&&’

Answer 1

This is the most vexing parse:

Derived c(std::array<double, 3>());

is a declaration of a function c which returns a Derived and takes one unnamed argument of type pointer to function that takes no argument and returns std::array<double, 3>. Therefore Derived d(c) tries to call Derived constructor from the function c. This is what GCC is saying here:

main.cpp: In instantiation of ‘Derived::Derived(Args&& ...) [with Args = {Derived (&)(std::array<double, 3ul> (*)())}]’:

Try this:

Derived c{std::array<double, 3>{}};